blown in dense pack cellulose is one of the most cost effective ways there is to add insulation. Whether it is sufficient to your needs depends a lot on where you live-- you can get to higher R values with foam, but at a substantial cost in dollars and in having to open up walls for application. In the old part of our house we achieved a very good result by wrapping the exterior in 2" of rigid foam, rigorous air sealing, and then blown in cellulose in the 2x4 walls. That. of course involved replacing all the windows and siding, something that was needed anyway on our case.